Electronic Devices Circuits Khurram Bughio Lecturer Electrical Engineering

Electronic Devices & Circuits Khurram Bughio Lecturer Electrical Engineering, MUCET

Semi Conductor Fundamentals

Electronic Devices • What is difference between Electrical devices and electronic devices • Diode, Transistor, Amplifiers & ICs • Electronic Devices are made of semiconductor material. • Conductor / Insulator • It is important to understand the basic atomic structure of semi-conductors in order to understand its behavior.

Atomic Structure • An atom is the smallest particle of an element that has all the characteristics of the element. • 109 different elements • Atom is like solar system • A central nucleus surrounded by orbits

1. Electron 2. Proton 3. Neutron • Electron: - vely charged particle that orbits around the nucleus • Proton: + vely charged particle in nucleus • Neutron: has no charge and present in nucleus. • • # of protons = Atomic number # of protons + # of neutrons = Atomic Weight Each element has unique atomic number Elements are arranged in a periodic table

Electrons, shell & orbits • Electrons orbit at discrete distances from the nucleus. • Electrons have different energies depending upon their distance from the nucleus. • Electrons near the nucleus have less energy and energy is increased as the distance is increased from nucleus. • Each discrete distance is known as orbit. • Orbits are grouped into energy bands called shells. • Each shell has a fixed minimum number of electrons at permissible energy levels (orbits) • Difference in energy within shells is much smaller as compared to the difference in energy between shells.

• The shells are designated as 1, 2, 3 …. . n • K, L and M are also used for shells. • Figure shows the concept of energy band. • First shell with one energy level and second shell with two energy levels • Energy increases as distance from nucleus increases.

Valence Electrons • Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus. • This is because the force of attraction between the positively charged nucleus and the negatively charged electron decreases with increasing distance from the nucleus. • Electrons with the highest energy exist in the outer-most shell of an atom and are relatively loosely bound to the atom. • This outermost shell is known as the valence shell and electrons in this shell are called valence electron. • These valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties.

Ionization • When an atom absorbs energy from a heat source or from light, for example, the energies of the electrons are raised. • The valence electrons possess more energy and are more loosely bound to the atom than inner electrons. so they can easily jump to higher orbits within the valence shell when external energy is absorbed. • If a valence electron acquires a sufficient amount of energy, it can actually escape from the outer shell and the atom's influence. • The departure of a valence electron leaves a previously neutral atom with an excess of positive charge (more protons than electrons). • The process of losing a valence electron is known as ionization. and the resulting positively charged atom is called a positive ion. • For example. the chemical symbol for hydrogen is H. When a neutral hydrogen atom loses its valence electron and becomes a positive ion, it is designated H+. • The escaped valence electron is called a free electron. When a free electron loses energy and falls into the outer shell of a neutral hydrogen atom, the atom becomes negatively charged (more electrons than protons) and is called a negative ion, designated H-.

The number of Electrons in a shell • The maximum number of electrons (Ne) in each shell of an atom can be calculated by the formula Ne = 2 n 2 Where n is the shell number • • • For shell 1 For shell 2 For shell 3 For shell 4 For shell 5 Ne = 2(1)2 = 2 electrons Ne = 2(2)2 = 8 electrons Ne = 2(3)2 = 18 electrons Ne = 2(4)2 = 32 electrons Ne = 2(5)2 = 50 electrons • All shells in a given atom must be completely filled with electrons except the outer (valence) shell.

Conductor • A conductor is a material that easily conducts electrical Current. • The best conductors are single-element materials, such as copper, silver, gold, and aluminum, which are characterized by atoms with only one valence electron very loosely bound to the atom. • These loosely bound valence electrons can easily break away from their atoms and become free electrons. • Therefore, a conductive material has many free electrons, that when moving in the same direction, make up the current.

Insulator • An insulator is a material that does not conduct electrical current under normal conditions. • Most good insulators are compounds rather than single-element materials. • Valence electrons are tightly bound to the atoms; therefore, there are very few free electrons in an insulator.

Semi conductor • A semiconductor is a material that is between conductors and insulators in its ability to conduct electrical Current. • A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. • The most common single-element semiconductors are silicon. germanium, and carbon. • Compound semiconductors such as gallium arsenide are also commonly used. • The single-element semiconductors are characterized by atoms with four valence electrons.

Energy Bands • Valence shell of an atom represents a band of energy levels and that the valence electrons are confined to that band. • When an electron acquires enough additional energy, it can leave the valence shell, become a free electron, and exist in what is known as the conduction band. • The difference in energy between the valence band the conduction band is called an energy gap. • This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band. • Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom.

• Figure shows energy diagrams for insulators, semiconductors, and conductors. • Notice in part (a) that insulators have a very wide energy gap. • Valence electrons do not jump into the conduction band except under breakdown conditions where extremely high voltages are applied across the material. • Part (b), semiconductors have a much narrower energy gap. • This gap permits some valence electrons to jump into the conduction band become free electrons. • Part (c) illustrates, the energy bands in conductors overlap. In a conductive material there is always a large number of free electrons.

Semiconductor vs Conductor

Silicon & Germanium

Conduction in Semiconductors • The electrons of an atom can exist only within prescribed energy bands. • Each shell around the nucleus corresponds to a certain energy band is separated from adjacent shells by energy gaps. in which no electrons can exist. • Figure shows the energy band diagram for an unexcited (no external energy such as heat) atom in a pure silicon crystal. • This condition occurs only at a temperature of absolute 0 Kelvin.

Conduction Electrons and Holes • An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons. • Free electrons are also called conduction electrons. • This is illustrated in the energy diagram of Figure (a) and in the bonding diagram of Figure (b)

• When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal. • This vacancy is called a hole. • For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating what is called an electron-hole pair. • Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band. • To summarize, a piece of intrinsic silicon at room temperature has, at any instant, a number of conduction-band (free) electrons that are unattached to any atom and are essentially drifting randomly throughout the material. • There is also an equal number of holes in the valence band created when these electrons jump into the conduction band. • This is illustrated in Figure

Electron & Hole Current • When a voltage is applied across a piece of intrinsic silicon, thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attracted toward the positive end. • This movement of free electrons is one type of current in a semiconductor material and is called electron current.

• Another type of current occurs in the valence band, where the holes created by the free electrons exist. • Electrons remaining in the valence band are still attached to their atoms and are not free to move randomly in the crystal structure as are the free electrons. • However, a valence electron can move into a nearby hole with little change in its energy level. thus leaving another hole where it came from. • Effectively the hole has moved from one place to another in the crystal structure. as illustrated in Figure. • This is called hole current.

N–type & P-type Semiconductors • The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductive material. • This process is called doping • Doping increases the number of current carriers (electrons or holes). • The two categories of impurities are – n-type – p-type.

N-Type Semiconductor • To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added. • These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb). • As illustrated in Figure, each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms. • Four of the antimony atom's valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron. • This extra electron becomes a conduction electron because it is not attached to any atom. • Because the pentavalent atom gives up an electron, it is often called a donor atom. • The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. • A conduction electron created by this doping process does not leave a hole in the valence band because it is in excess of the number required to fill the valence band.

• Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an ntype semiconductor (the n stands for the negative charge on an electron). • The electrons are called the majority carriers in n-type material. • Although the majority of current carriers in n-type material are electrons, there also a few holes that are created when electron-hole pairs are thermally generated. • These holes are not produced by the addition of the pentavalent impurity atoms. • Holes in an n-type material are called minority carriers.

P-type semiconductor • To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. • These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga). • As illustrated in Figure, each trivalent atom (boron. in this case) forms covalent bonds with four adjacent silicon atoms. • All three of the boron atom's valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added. • Because the trivalent atom can take an electron, it is often referred to as an acceptor atom. • The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. • A hole created by this doping process is not accompanied by a conduction (free) electron.

• Since most of the current carriers are holes. silicon (or germanium) doped with trivalent atoms is called a p-type semiconductor. • Holes can be thought of as positive charges because the absence of an electron leaves a net positive charge on the atom. • The holes are the majority carriers in p-type material. • Although the majority of current carriers in p-type material are holes, there also a few free electrons that are created when electron-hole pairs are thermally generated. • These free electrons are not produced by the addition of the trivalent impurity atoms. • Electrons in p-type material are the minority carriers.

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